1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and more particularly to a spread illuminating apparatus used with a liquid crystal display.
2. Description of the Related Art
A liquid crystal display (hereinafter referred to as LCD), which is small in power consumption, low in profile, and light in weight, is heavily used in electric products such as a personal computer (hereinafter referred to as PC), a cellular phone, and the like, and is increasingly demanded.
Since a liquid crystal of the LCD does not emit light by itself, the LCD requires an illuminating means to radiate light on the liquid crystal when used in a place where sunlight or interior lighting is not fully available.
A PC, particularly notebook-type PC, and a cellular phone are required to be lower in profile and smaller in power consumption, and the requirements are fulfilled by a spread illuminating apparatus of side light type.
A conventional spread illuminating apparatus of side light type is shown in FIG. 13. In FIG. 13, a spread illuminating apparatus 1′ generally comprises a light conductive plate 2 made of a light transmissible material, and a bar-like lamp 5 disposed along and close to an end face 8 of the light conductive plate 2. Light emitted from the light lamp 5 is introduced into the light conductive plate 2 and directed to an LCD (not shown) disposed under the light conductive plate 2.
The lamp 5 comprises a light conductive bar 3 made of a light transmissible material, and two spot-like light sources 4, 4 (for example, light emitting diodes) disposed facing respective end faces 6, 7 of the light conductive bar 3. The light conductive bar 3 has an optical path conversion means 12 formed on a side face thereof opposite to a side face 9 facing the end face 8 of the light conductive plate 2. The optical path conversion means 12 comprises, for example, a plurality of grooves shaped triangular in section, and is adapted to guide light, which is emitted from the light source 4 into the light conductive bar 3, toward the end face 8 of the light conductive plate 2 in a substantially uniform manner. The light conductive bar 3 is disposed with its side face 9 facing the end face 8 of the light conductive plate 2 with a predetermined distance therebetween.
The light conductive bar 3 has a light reflection member (frame) 13 substantially U shaped in section and disposed therearound. The light reflection member 13 covers the longitudinal faces of the light conductive bar 3 except the side face 9 facing the light conductive plate 2.
The light conductive plate 2 has a light reflection pattern 19 formed on its upper face 15, and has a plain surface on its lower face 26. The light reflection pattern 19 has a stair-like configuration in section, comprising a plurality of small surfaces 17 having its longitudinal direction parallel to the length of the light conductive bar 3, and a plurality of large surfaces 18 each present between two adjacent small surfaces 17. The light reflection pattern 19 is adapted to guide light, which is emitted from the lamp 5 into the light conductive plate 2, toward the LCD (not shown) disposed under the light conductive plate 2. The light reflection pattern 19 may alternatively comprise a plurality of grooves, and a plurality of flat portions present between two adjacent grooves.
In the front-lighting system where an illuminating apparatus using the light reflection pattern 19 is disposed over the front face of a reflection type LCD element, when light reflected at the reflection type LCD element passes through the light reflection pattern 19, an interference fringe (moire pattern) appears, which is formed by the striping generated due to a difference in light outgoing efficiency resulting from a difference in refractive index between at the small surface 17 and at the large surface 18, and by the arrangement of the mosaic pattern (cell boundary) of crystal cells constituting pixels of the LCD, and which is detrimental to the observation of the image on the display. The moire pattern is closely related with the configuration of the light reflection pattern 19, specifically, the dimensions and inclinations of the small surfaces 17 and the large surfaces 18, or the like.
Also, there appears a light and dark striping of another kind different from the above described moire pattern. The light and dark striping is peculiar to the front-lighting system, and is characterized in that its location and striping interval change according to the position of the observer's eye. It has become apparent that the light and dark striping is generated by reflected light due to Fresnel reflection at the lower face of the light conductive plate opposite to the face provided with the light reflection pattern, that is, reflected light caused by the difference in refractive index between the light conductive plate and the air. It has been known that this reflected light has an adverse influence on the contrast characteristics of the display device, and the light conductive plate normally has non-reflective coating applied to its lower face for improving contrast.
The current non-reflective coating suppresses light reflection significantly but not down to 0% across the visible display area, normally allowing some 0.2% of light incident thereon to be reflected. This slight amount of reflected light generates the light and dark striping.